This invention relates generally to apparatus and methods for measuring the differential pressure of fluid internal and external to a pipe, and more particularly to apparatus and methods for measuring the differential pressure between the mud internal to the drill collar and the mud in the borehole external to the drill collar during drilling.
Various measuring-while-drilling (MWD) techniques for telemetering data representing various monitored downhole conditions during the drilling of a well have been suggested. For example, U.S. Pat. Nos. 4,479,564 to Tanguy, 4,100,528 to Bernard et al., 4,103,281 to Strom et al., 3,309,656 (Godbey), 3,820,063 to Sexton et al., and 3,855,857 to Claycomb all disclose MWD systems using phase-shift-keyed (PSK) modulation for monitoring such useful conditions as direction of the hole, weight on bit, bit torque, temperature, natural gamma radiation, formation resistivity, and the predicted drill bit path. Another downhole condition, the monitoring of which is particularly useful to the driller is the differential pressure between the mud internal and external to the drill collar. The differential pressure may be used for monitoring surge and swab. Likewise, the differential pressure may be used for determining whether gas or liquid has intruded into the borehole such that the potential of a blowout exists, or whether fluids or sand are being forced into the formation such that hydrocarbon production could be jeopardized due to formation damage. Further, differential pressure may be used as an indication of the bit nozzle conditions and of drill collar washouts.
Apparatus and methods for measuring differential pressure are known in the art. Typically, two pressure transducers are used, with a first pressure transducer measuring the internal (drill collar) pressure and a second pressure transducer separately measuring the external (annulus or borehole) pressure. Such an arrangement is seen in one embodiment of U.S. Pat. No. 4,297,880 to E. Berger. Among the many problems of the two pressure transducer arrangement is that resolution is poor because the two transducers must a have large full scale (0 to 22,000 psi) to obtain a differential pressure of a much smaller full scale (0 to 5000 psi). Thus, two big values are being subtracted to obtain a small value. In addition, the cost, packaging, and reliability of the two transducer arrangement is disadvantageous.
While a single pressure transducer arrangement for measuring differential pressure is suggested by second embodiments (FIGS. 3 and 4) of the aforementioned Berger U.S. Pat. No. 4,297,880, it will be appreciated that such an arrangement has various disadvantages. A first disadvantage is the possibility of inaccurate measurement due to axial and torque forces on the tool and the application of the mud at orthogonal angles to the transducer. In addition, the Berger arrangement causes the transducer to be directly subjected to both the internal and external mud and could result in undesired mud leakage which could cause tool failure and/or could interfere with the MWD signal.
Of further interest for its geometry is the aforementioned Claycomb U.S. Pat. No. 3,855,857 which has a thin-walled sleeve having a reduced diameter mid-portion. The apparatus disclosed by Claycomb, however, is particularly arranged to measure axial forces on the drill bit and not the differential mud pressure. In fact, as indicated in Col. 6 line 57 through Col. 7, line 38, of the Claycomb patent, care is taken to isolate the pressure transducer from the mud so that the mud pressure will not affect the desired measurements.
Given the tools of the art, it will be appreciated that an accurate and simple differential pressure measurement arrangement for an MWD tool would be most desirable.